The present invention relates to a method and a device for determining parameters describing changes in a technical system caused by ageing, particularly for a system including one or more electric motors, usage-dependent performance quantities being determined in the system. The present invention also relates to a computer program and a computer program product, in each case having a program-code arrangement, in order, when they are executed on a computer or a computing unit, to permit ageing parameters of a system to be determined.
The determination of changes in a system caused by ageing is important for an adaptive control of various electromechanical components as are encountered, for example, in a motor vehicle (e.g., fuel-injection systems), and can also be used to estimate the wear and tear and the remaining service life of motors in electric tools or generators, for example.
German Published Patent Application No. 197 16 520 describes a device for determining performance quantities of electric motors, in which certain usage-dependent performance quantities such as temperature, number of motors start-ups and operating hours are determined and stored. Using an external readout device, the recorded data can be read out and interpreted. Conclusions can then be drawn as to whether after the service life of a product has expired, the electric motor used in the product can be further used in a new product.
German Published Patent Application No. 195 16 481 describes a device for acquiring, storing and outputting data of a control unit in a motor vehicle, with which, in addition to the operating time of the control unit, further data are acquired which are able to give information with respect to the probability of malfunction or the future reliability of the control unit. Among these data are, for example, extreme temperature values, as well as extreme voltage values and their duration. The document indicated does not describe how conclusions about the probability of malfunction or reliability of the control unit are drawn from the specified data.
Parameter-estimation methods also exist based on neural networks or on extended Kalman filters which, till now, have been used successfully, inter alia, for identifying friction in actuating systems. These methods can only be used conditionally for predicting wear conditions, for identifying wear parameters and for estimating the remaining service life of products. When the identification task is multi-dimensional, they often fail, and guarantee no or not a sufficiently fast convergence.
Therefore, the intention of the present invention is to indicate an identification method, which can be implemented on-line, for determining changes in parameters describing a technical system caused by ageing, as well as a device suitable for carrying the method into effect and a corresponding computer program (product).
Using the method of the present invention, parameters which describe changes in a system caused by ageing can be calculated within the system during the entire operating time of the system. To that end, with the aid of a wear and tear model, first of all a correlation is produced between the usage-dependent and measurable performance quantities detected in the system, and the ageing-inherent, but not directly measurable parameters characterizing the wear and tear or the ageing. From this wear and tear model, the parameters describing the changes in the system caused by ageing, or quantities derived therefrom, can then be calculated, which are made available in the system.
A device of the present invention for determining parameters which describe changes caused by ageing in a technical system has a unit to which the usage-dependent performance quantities determined in the system are supplied. Taking as a basis the aforesaid wear and tear model, the ageing parameters or quantities derived therefrom are then calculated in this unit within the system. The described unit can be regarded as a component of a computer orxe2x80x94particularly in the case of motor vehiclesxe2x80x94of a control unit. In the following, the same considerations can also be brought to bear on all parts subject to wear and ageing, whether in vehicles, airplanes or even in the producer-goods and/or consumer-durables sector, etc. Thus, on this matter, the exemplary embodiments are not to be understood as restriction of the subject matter according to the present invention.
The indicated parameters or the quantities derived therefrom can be calculated continually or at pre-definable time intervals. Depending upon the field of application and system class, the result of the parameter calculation can be displayed visually, by output of a numerical value or a warning, or acoustically. However, the result can also be made available to the control unit of the system to permit control of the system adapted to the operating time or to ageing processes.
Frequently, the indicated wear and tear model can be represented in the form
f=f(p,x(t),a(t))=0xe2x80x83xe2x80x83(1)
the generally vectorial quantities f, p, a and x representing the following:
x(t) describes usage-dependent and measurable performance quantities of a system or product, such as use-dependent internal state variables, e.g. temperature, current, power consumption and/or acceleration, as well as number of switch-on cycles and total operating time.
a(t) describes usage-dependent, not directly measurable parameters, which, because of wear and ageing with increasing use of a system or product, change with time t, such as parameters for describing catalysis properties of a catalytic converter, absorption properties of an absorber or filter, or even the increasing probability of malfunction or the decreasing remaining service life of a product. The knowledge of these parameters allows for assessing product wear and for adaptive tuning of control algorithms in control units.
p describes usage-independent, system-immanent, not directly measurable parameters which describe substance properties of a system or product class (system constants). Since for individual products these parameters p can be subject to fluctuations depending on manufacture, they represent average values for the class of products considered.
A system in which the acquired, usage-dependent performance quantities can be retrieved external to the system is described in German Published Patent Application No. 197 16 520 indicated. In one embodiment of the present invention, for a plurality of systems which belong to one common class, the respective acquired performance quantities are retrieved externally after a specific usage time which can be different from system to system. For example, this can take place at regular time intervals, or always when the system or product is brought to a service place for inspection, repair or waste disposal. From the acquired performance quantities xk(t) of the k-th system, after a usage time tk, optionally after additional measurements of the wear and tear of the used products, ageing parameters ak(tk) can be determined which describe ageing-inherent changes in the system that are not directly measurable. By inserting ascertained performance quantities xk(t) and the determined ageing parameters ak(tk) into wear and tear model fk of a system indicated in equation (1), a parameter vector p, which includes P usage-independent system-immanent parameters (A, B, . . . ), can now be calculated as average value of the system class.
A device suitable for determining the wear and tear model of a system class has a readout unit through which stored performance quantities are read out for a number of K systems of a system class, and, together with the ageing parameters determined therefrom, are supplied to a computing unit. The parameter vector p is then calculated in the computing unit in order to obtain a completely determined wear and tear model.
In particular, for each k-th product, a wear and tear model
xe2x80x83fk=f(p,ak(tk),xk(t))=0, k=1, . . . ,K,xe2x80x83xe2x80x83(2)
can be formulated, and the parameter vector p=p* can be determined as solution of the equation system. For K greater than P, a redundant, generally non-linear equation system is formed which can be solved by a suitable optimization method that, in particular, should supply good approximate values for K greater than  greater than P. The result is an approximation for the parameter vector p=p* for the class of systems or products considered.
At this point, the ascertained parameter vector p=p* can be inserted into the wear and tear model f of equation (1), so that the wear and tear model for the system or product class in question is completely known, at least in a good approximation. The wear and tear model can be stored in the system and calculated continuously during the service life, so that system-internal statements are possible concerning the wear or ageing.
In this context, in order to minimize the on-line computing time, it may be useful to solve the wear and tear model directly according to ageing parameters a(t) with the aid of mathematical methods such as power series statements or recursive methods.
After implementing the method of the present invention in the system, the instantaneous values of performance quantities x(t) can be read in afresh in each measuring cycle for determining ageing parameters a(t) and can be overwritten again in the next measuring cycle, so that it is not necessary to store these values over the entire operating time.
All calculations described can expediently be carried out by a computer program which is executed on a suitable computing unit. For example, for continuous on-line determination of the ageing parameters, a unit can be provided in the system to which the acquired, usage-dependent performance quantities are supplied that are then evaluated in a computing unit. The corresponding computer program works with a predetermined wear and tear model. This wear and tear model is expediently also determined by a computer program, which can be advantageous, particularly given the occurrence of non-linear equation systems. The computing unit for this is accommodated in a device external to the system, in order to be able to process data from a totality of systems.
The computer programs can be stored on suitable storage media such as EEPROMs, flash memories, but also CD ROMs, diskettes or hard disk drives.
The recording and evaluation, according to the present invention, of historical load patterns permits the detection of changes in a total system or its components (e.g., motor vehicle or electric tool) caused by ageing, and therefore, in the next step, for instance, an adaptation of algorithms in control units to new conditions, an early warning in the case of safety-critical systems, the specification of a degree of wear for determining servicing intervals, or an ageing-dependent reuse of used components, and more of a similar nature.
If reliable wear and tear models are found which cover all conceivable defects, the method of the present invention makes it possible to make reliable statements about the degree of wear and the remaining service life at any point of time as desired, without the system or its components having to be inspected in so doing. The frequently used MTBF (mean time between failures) data cannot supply this information if the usage intensity is not known a priori. If, when using the method of the present invention, a specific remaining service life (taking into consideration a safety factor) or a critical degree of wear is reached, a system-internal control unit calculates the point of time for a product or component exchange and recommends it automatically. This can be useful for motor vehicles, particularly in the case of service-station visits within the framework of service intervals, as well.
For example, after taking back products from pilot production and test series, or after equipment defects, the manufacturer on its part receives valuable information about actual load profiles which supply valuable information for the quality assurance and for determining MTBF.